1. Field of the Invention
The present invention relates to systems and techniques for producing laser pulses in rapid succession, and to such laser systems used in the field of Particle Image Velocimetry (PIV).
2. Description of Related Art
In PIV, laser pulses are directed in rapid succession into a fluid flow, which in some cases includes tracer particles. A camera is used to capture images of the fluid flow during the pulses. The images captured can be analysed to determine characteristics of the flow. Often, a single exposure by the camera is used to capture the image during two pulses. For some embodiments, the laser pulses must be generated in rapid succession. Also, the laser pulses should be substantially equal in power and duration to simplify the analysis of the captured image or images. Finally, it is important for the laser pulses to illuminate the same space in the field of the camera during the successive pulses, because changes in position of the illumination complicate analysis of movement of the particles in the flow.
FIG. 1 illustrates a prior art PIV system, including first laser 101 and second laser 102 which are used to produce pulses (schematically pulses 101A, 102A) of laser light in rapid succession. Pulses from the first laser 101 and second laser 102 are merged to a single beam line and used as a pair (schematically pulse pair 124) to form an illumination sheet 125 in a fluid flow 140. Camera 133 acquires an image 132 of particles in the illumination sheet 125. In this example, a polarized output pulse 103 from first laser 101 is directed to mirror 110, which reflects it towards dichroic polarization splitter 111. Output beam 104 from second laser 102 has polarization rotated by ninety degrees from the polarization of the output pulse 103, and is likewise directed to the dichroic polarization splitter 111, where its path is co-located in laser beam path 105 with the path of output pulse 103 from first laser 101. In some embodiments, the lasers 101 and 102 comprise diode pumped, Nd:YAG solid state lasers producing output pulses in a primary wavelength of 1064 nanometers. In these embodiments, a harmonic generator (not shown) may be placed in path 105, to convert the 1064 nanometer pulse to a visible wavelength, such as 532 nanometers. Path 105 directs the pulses through spherical lens 115 to prism 120. Prism 120 directs pulses on laser beam path 105 to cylindrical lens 121, which forms the pulses into pulsed illumination sheet 125 in the fluid flow 140.
Illumination sheet 125 illuminates particles 131 within fluid flow 140, and the particles thus illuminated form an image 132 within camera 133. Particle images thus acquired, are processed by computer where a PIV analysis can be performed.
Commonly, pulses from two separate lasers are required in Particle Image Velocimetry applications, because the time between pulses is shorter, and the energy of the pulses is higher, than can be practically generated using a single gain medium. A controller, typically implemented using a computer, sets a controllable time delay between the separate lasers, and sets the power of the output pulses. The optical components used to create the illumination sheet must be carefully aligned, so that the pulses from the two lasers illuminate substantially the same space in the flow. Any misalignment of the illuminated spaces is directly reflected in the captured image, and complicates the analysis of the image. Also, the energy of the pulses should be substantially the same or precisely controlled, so that the captured images of the two pulses can be more readily processed.
As shown in FIG. 2, one prior art system employs two IR laser resonators. A first laser 101 has a resonant cavity including first mirror 231, a first laser gain module 230, a first Q-switch 233, a first output coupling mirror 234. A second laser 102 has a resonant cavity including a second mirror 251, a second laser gain module 250, a second Q-switch 253, a second output coupling mirror 254. A laser controller 240 is coupled to the laser gain modules and Q-switches of the first and second lasers, and can adjust parameters of the system, including the time delay between the pulses from the first and second independent lasers.
In the embodiment of FIG. 2, the first laser output beam 260 is reflected by mirror 255 to polarizer 256, where it is combined with the second laser output beam 261 which has passed through half-wave plate 259. The beam paths are merged at polarizer 256 into a co-located beam path 262. The beam path 262 is directed to harmonic generator 258, which converts the pulses on beam path 262 to output pulses on beam path 264 having a visible wavelength.
The output beam 264 thus formed can be utilized to form an illumination sheet for use in PIV measurements in a manner similar to that shown in FIG. 1.
The technique of externally combining pulses generated by two independent IR lasers has several shortcomings. For example, this arrangement is highly sensitive to the correct mechanical alignment of the lasers and the optics in the beam paths.